Recently, there has been an increasing interest in energy storage technology. As the application fields of energy storage technologies have been extended to cellular phones, camcorders, notebook computers, PCs and electric cars, efforts have been increasingly been made towards the research and development of electrochemical devices capable of charging and discharging, particular a lithium secondary battery.
The secondary battery commonly uses a porous separator which is, however, subject to severe thermal shrinkage at a temperature of 100° C. or higher in view of their material characteristics and production processes including elongation. This thermal shrinkage behavior may cause a short circuit between a cathode and an anode. Also, when the battery undergoes a rapid rise in voltage upon overcharge owing of various causes, including the abnormal operation of a charger, excessive amounts of lithium is disintercalated in a cathode and intercalated in an anode, depending on state of charge, making both electrode of the cathode and the anode be thermally unstable. In this case, an organic solvent in an electrolyte solution is discomposed to cause rapid exothermic reaction resulting in thermal runaway, which may cause great damage in battery stability. Such an overcharge may cause a local internal short circuit and an intensive temperature rise occurs in the position of the local internal short circuit. In order to prevent the internal short circuit, the lithium secondary battery should have a separator with good heat-resistance, particularly a minimized shrinkage rate, at a high temperature. In addition to such properties, the separator should also be made of a thin film so as to achieve the miniaturization of a battery pack and minimize electrical resistance, and is required to have uniform and high porosity so as to improve charging/discharging efficiency and cycle characteristics.
Conventional separators are generally a single- or multi-layered thin film made of a polyolefin. However, these separators fail to have heat-resistance, particularly shrinkage rate at a high temperature enough to prevent an internal short circuit stably, and they are difficult to have uniform and high porosity due to a limit in their preparation method.